High-quality germanium (Ge) on silicon (Si) continues to be of interest for many notable applications. Specifically, Ge on Si can be used as substrates for III-V integration to fabricate multijunction solar cells, high-carrier-mobility field effect transistors, high-speed heterojunction bipolar transistors, infrared detectors, etc. However, the main difficulty in achieving high-quality Ge is the 4.2% lattice mismatch and 116% thermal expansion coefficient (TEC) mismatch between Ge and Si. The lattice mismatch results in high threading dislocation densities (TDDs) on the order of 108-109 cm−2, while the TEC mismatch leads to microcracks in Ge films thicker than 5 μm as the substrate cools from a growth temperature to room temperature. Conventional attempts to reduce the defect density in Ge on Si include using graded GexSi1-x buffer layers, cyclical thermal annealing, strained-layer blocking, small-area mesas, and aspect ratio trapping.
Conventional methods for forming Ge on Si include exposure of an ultrathin SiO2 layer on Si to a Ge flux, which results in a nucleation of over 1011 cm−2 epitaxial Ge dots. The Ge dots are shown to nucleate within tiny openings in the SiO2 layer (see Shklyaev, et. al, Phys. Rev. B 62, 2000, 1540). Ge beam and Si substrate participate in the decomposition reaction of SiO2 to form volatile byproducts of SiO and GeO (see Leonhardt, et. al., Thin Solid Films 518, 2010, 5920). Tromp et al. observed that the erosion of SiO2 does not occur uniformly, but rather at random locations that likely involve defects at the Si—SiO2 interface (see Tromp, et. al., Phys. Rev. Lett. 55, 1985, 2332). One possible defect is dangling bond defects, known as PbO detectable by electron spin resonance spectroscopy. The reported density of PbO interfacial defects is on the order of 1012 cm−2, which is close to the number of openings in the SiO2 where Ge nuclei form (see Leonhardt, et. al., Thin Solid Films 518, 2010, 5920; Li, et. al., J. Appl. Phys. 98, 2005, 73504; Li, et. al., Appl. Phys. Lett. 85, 2004, 1928; and Shklyaev, et. al., Phys. Rev. B 62, 2000, 1540).
Although the technique of using Ge nucleation through nanometer openings in SiO2 generates low TDD Ge films on Si, a high stacking fault (SF) density of about 5×107 cm−2 is generated at the surface of the Ge film (see Leonhardt, et. al., Thin Solid Films 518. 2010, 5920). Upon polishing the coalesced Ge film with a non-slurry-based solution, these SFs manifest themselves as protruding lines along <110> directions raised 1-2 nm above the surrounding, polished Ge film surface. These SFs cause significant surface roughness and lead to non-radiative recombination centers in GaAs that is subsequently grown on Ge/SiO2/Si virtual substrates (see Leonhardt, et. al., Thin Solid Films 518, 2010, 5920 and Cederberg, et. al., J. Cryst. Growth 312, 2010, 1291).
Thus, there is a need to overcome these and other problems of the prior art and to provide methods for reducing or eliminating defects when forming semiconductor layers/devices by nucleation and coalescence.